<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        let num = prompt('请输入一个数字: ');
        num = num < 10 ? '0' + num : num;
        alert('补零后的数字是: ' + num);
    </script>
</body>
</html>